A pipe fitting, as shown in FIG. 1, is disclosed in German Patent DE202005008109. The pipe fitting comprises a body 11, a jacket 12, a seal ring 13 and a plastic check ring 14, which are clamped to establish the seal joint of polyethylene/aluminum/polyethylene composite pipes. However, the assembly of this type of pipe fitting is of relative high requirements, such as the ends of the pipe 4 needs to be cut even and a full-round action should be performed. An annular convex portion 142 of the plastic check ring 14 forms a close fit with the annular groove 111 on the core of the body 11, therefore the pipe is relative hard to be inserted into the fitting, owing to the fact that the jacket 12 is eccentric to the core of body 11. The pipe should be inserted into the fitting with carefulness, for the outer diameter of seal ring 13 is larger than the core of body 11, or else the seal ring 13 would be pushed out of the core of body 11 easily, which affects the sealing performance greatly. During the clamping process, the end face 141 of plastic check ring 14 is positioned with respect to the clamping tool, thereby, an imprecise positioning that affects clamping result may occur. Meanwhile, the seal ring 13 is of a certain compression after the pipe 4 is inserted into the pipe 4 without being clamped, so that when the hydraulic pressure of pipeline system is checked, leakage would not appear. As a result, a leakage due to worker's neglect of assembly may not be checked out. Once the clamping of the fitting is neglected, a great leakage that leads to unwanted economic loss may occur after a long time.